Given a + 2B + 3C = 20, a + 3B + 5C = 31, then the value of a + B + C is () A. 6B. 7C. 8D. 9

Given a + 2B + 3C = 20, a + 3B + 5C = 31, then the value of a + B + C is () A. 6B. 7C. 8D. 9

∵ a + 2B + 3C = 20 (1), a + 3B + 5C = 31 (2). From (2) - 1, we get B + 2C = 11, ∵ B = 11-2c (3). Substituting (3) into (1), we get a = - 2 + C, ∵ a + B + C = - 2 + C + 11-2c + C = 9

Given a + 2B + 3C = 20, a + 3B + 5C = 31, then the value of a + B + C is ()
A. 6B. 7C. 8D. 9

∵ a + 2B + 3C = 20 (1), a + 3B + 5C = 31 (2). From (2) - 1, we get B + 2C = 11, ∵ B = 11-2c (3). Substituting (3) into (1), we get a = - 2 + C, ∵ a + B + C = - 2 + C + 11-2c + C = 9

(a-b) (a-2b) + (a-2b) (a-3b) - 2 (a-3b) (a-4b), where a = - 4, B = 1
First simplify and then evaluate,
I figured it out as 5ab-16b ^ 2-2a ^ 2,
The answer is ^ 2 = - 6ab
Why?
It's better to do it step by step
For the first floor, step 3, what is (1 + 1-2) a ^ 2 and why

Three and two to one ratio two and three to one ratio

=7/2：7/3
=7/2÷7/3
=7/2×3/7
=3/2
=3：2

Simplification ratio, 1,6:2 / 3

39 to 20

Simplify the following ratio: 2 and 1 / 3: 3 and 1 / 2
1:3:1:4

2 and 1 / 3: 3 and 1 / 2
=Seven out of three: seven out of two multiplied by six at the same time
=14: Divide both sides of 21 by 7
=2:3
1:3:4:1:5 times 60
=20：15：12

-(+ 2 / 3) reduced to?

-(+2/3)=-2/3

Six out of seven, three out of four

The least common divisor is 28, which is multiplied by 28 to get 24:21

Simplification; 5 / 12 / 4 / 3 / 6 / 5

5 out of 12: 3 out of 4: 5 out of 6
=5 out of 12: 9 out of 12: 10 out of 12
=5:9:10

1 / 3 to 7 / 6 to 2

1:3:7:6:2
=(1 / 3 * 21): (6 / 7 * 21): (2 * 21)
=7：18：42
Thank you